1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Base stations in wireless communication systems provide wireless connectivity to users within a geographic area, or cell, associated with the base station. In some cases, the cell may be divided into sectors that subtend a selected opening angle (e.g., three 120° sectors or six 60° sectors) and are served by different antennas. The wireless communication links between the base station and each of the users typically includes one or more downlink (DL) (or forward) channels for transmitting information from the base station to the mobile unit and one or more uplink (UL) (or reverse) channels for transmitting information from the mobile unit to the base station. Multiple-input-multiple-output (MIMO) techniques may be employed when the base station and, optionally, the user terminals include multiple antennas. For example, a base station that includes multiple antennas can concurrently transmit multiple independent and distinct signals on the same frequency band to same user or multiple users in a cell/sector. MIMO techniques are capable of increasing the spectral efficiency of the wireless communication system roughly in proportion to the number of antennas available at the base station.
Conventional MIMO techniques require that the multiple antennas be co-located with the coordinating base station so that the relative timing delays introduced by propagation from the base station to the antenna are negligibly small. For example, the multiple antennas associated with a base station (BS) are typically configured so that the antennas are less than about 10 m from the base station. Typically these connections between the BS and its antennas are broadband high-grade RF cables. Furthermore, the relative differences in the distances from the BS to each of its antennas is very small. Thus the actual value of the relative time delay differences are usually significantly smaller than the absolute delay between the BS and an antenna because each antenna is deployed approximately the same distance from the base station. When the relative time delay for signals transmitted between the antennas and the base station is negligibly small, the signals transmitted from the base station to the antennas and then over the air interface to MS on DL may be phase aligned easily so that they can be coherently combined at the receiver, e.g., the mobile unit. Constructive and/or destructive interference of coherent radiation from the multiple antennas can therefore be used to amplify the signal in selected directions and/or null the signal in other directions. Processing of the coherent signals may also be used to minimize the mutual interference between multiple transmitters. Similarly on UL, signals received from multiple antennas can be combined to maximize signal strength, maximize SINR, detect multiple signals simultaneously through well-known algorithms such as MRC (maximum ratio combining), MMSE (minimum mean squared error), and MLSE (maximum likelihood sequence estimator).
Conventional soft handoff techniques employ multiple BSs for both DL and UL to improve diversity but do not exploit joint spatial processing techniques that can yield much higher gains. Increasing the separation between antennas significantly can reduce the coherence between the signals transmitted and/or received by these antennas because of the potentially large and unpredictable relative time delays introduced by transmitting these signals to a central point for processing. For example, signals received (or transmitted) by antennas at two geographically separated base stations would need to be transmitted to (or received from) a central location using MS specific timing in a very tightly controlled and coordinated manner to achieve the gains this disclosure addresses. However, conventional backhaul links to the central location introduce large and/or variable timing delays that cause the signals carried over the backhaul links to become incoherent. Consequently, each base station independently decodes signals received over an uplink and transmits signals over the downlink. The demodulating and decoding processes at separate base stations are not coordinated and the signals transmitted by different base stations cannot be coherently combined. Similarly signals transmitted by conventional base stations are not coordinated to permit the signals to be coherently combined at the mobile units.